1. Field of Invention
This invention relates to coil arrangements for producing static magnetic fields, such as those used in NMR (nuclear magnetic resonance) tomographs.
2. Description of Prior Art
NMR spectroscopy is a uniquely effective means for enabling a user to observe matter microscopically, especially in the fields of condensed matter physics and chemistry. NMR spectroscopy uses a uniform static magnetic field and a weak RF magnetic field which weakly interact with a sample to be observed. The energy of the RF magnetic field is usually about 10.sup.-19 to 10.sup.-20 erg, which is much less than the energy for X-rays, which ranges from 10.sup.-8 to 10.sup.-9 erg.
Because of this feature, NMR spectroscopy has attracted much attention as a technique for observing living bodies without injuring them in medical applications. For example, application in clinical medicine in the area of NMR CT (NMR computerized tomography is being studied. This study was stimulated by a report that the relaxation time of protons constituting water molecules of malignant tumor is several times longer than the relaxation time for normal tissues (see Science, Vol. 171, p. 1151, 1971, R. Damadian).
An NMR tomograph which produces cross-sectional images of an object under examination, utilizing the NMR phenomenon, usually uses normally conducting magnets of the structure shown in FIG. 2 to produce a static magnetic field, because they are adapted for the human body and have good performance characteristics. In order to obtain images of high quality, the magnetic field homogeneity in the region to be observed is required to be as high as 10 ppm or the like. To achieve this, various designs for the structure of the coil arrangement have been heretofore proposed. One such design is disclosed in "Practical Method of Improving the Uniformity of Magnetic Fields Generated by Single and Double Helmholtz Coils," Rev. Sci. Instrum. 52(3), March 1981, pp 447-453, and is next described with reference to FIG. 2.
Referring to FIG. 2, four coils are arranged symmetrically to cancel out the error terms of the second and fourth orders and even of the sixth order, of the magnetic field. These are known as Helmholtz coils and are frequently used as normally conducting magnets of NMR tomographs. This prior coil system has heretofore been designed using the following criteria.
1. When the coils can be approximated by a concentrated current loop, the intervals between the coils are set as follows: EQU cos .theta..sub.1 =z.sub.1 /R.sub.0 =0.76506 EQU cos .theta..sub.2 =z.sub.2 /R.sub.0 =0.28523
The ratio between the ampere turns of the coils is EQU AT.sub.2 /AT.sub.1 =1.4660
Under these conditions, the error terms of the second, fourth and sixth orders of the magnetic field are all nill. Thus, an eighth order compensation coil arrangement is provided.
2. Double Helmholtz coils having a combination of rectangular cross sections of finite dimensions are designed as follows. EQU cos .theta..sub.1 =z.sub.1 /R.sub.0 =0.76506 EQU cos .theta..sub.2 =z.sub.2 /R.sub.0 =0.28523
The ratio between ampere turns of the coils is EQU AT.sub.2 /AT.sub.1 =1.46608 EQU a.sub.1 /a.sub.2 =0.67188
If the current densities through the coils are the same, then EQU b.sub.1 /b.sub.2 =1.01523
The factors are so set that EQU a.sub.1, a.sub.2, b.sub.1, b.sub.2 &lt;&lt;R.sub.0
By selecting the dimensions in this manner, the error terms of the second, and fourth orders of the magnetic field are both zero. Thus, a sixth order compensation coil arrangement is obtained.
3. The dimensions of double Helmholtz coils having rectangular cross sections of finite dimensions, are set as indicated in Table 1 so that an eighth order compensation coil arrangement is obtained. In Table 1, Na.sub.1, Nb.sub.1, Na.sub.2, Nb.sub.2 indicate relative number of turns when a.sub.1, b.sub.1, a.sub.2, b.sub.2 assume their optimum dimensions.
TABLE 1 ______________________________________ Example 1 Example 2 Example 3 Example 4 ______________________________________ b.sub.1 /2R.sub.0 0.04206 0.09386 0.07503 0.08116 z.sub.1 /R.sub.0 0.76598 0.76847 0.76346 0.76015 z.sub.2 /R.sub.0 0.28501 0.28519 0.28638 0.28700 Nb.sub.1 32 20 15 15 Na.sub.1 16 15 20 24 Nb.sub.2 25 20 21 23 Na.sub.2 30 22 21 23 .gamma..sub.8 -2.106 -2.101 -2.083 -2.080 ______________________________________
When an eighth order compensation coil arrangement is used, the magnetic field has the following relation: EQU B.sub.z (z,0)=B.sub.0 [1+.gamma..sub.8 (z/R.sub.0).sup.8 + . . . ](1)
The conventional coil arrangement for producing a static magnetic field is capable of setting up a magnetic field of a given high homogeneity. However, under any one or more of the above criteria (1), (2), and (3), the inner coils are lengthwise, because a.sub.2 /b.sub.2 .gtoreq.1. Thus, disadvantageously, the coils of this prior structure (a) have large dimensions (the maximum outer dimension of the coils is 1600 mm), (b) consume a large amount of electric power (of about 60 KW), and (c) are relatively heavy (weighing about 2,000 Kg). Also, as compared with these large values, the diameter C of the clear bore (see FIG. 2), that is limited by the smaller coils, takes a small value of about 700 mm.
Thus, the situation in the art is that conventional coil arrangements are largely unsophisticated with magnetic field requirements being produced by large and heavy coil arrangements that require large amounts of power to operate. Efficiency and reliability as well as cost leave much to be desired.